In a wireless network, in order to enter the network a mobile station normally has first to acquire signals, search for a suitable base station and then perform a random access to establish communication with that base station. The process of acquiring signals involves signal detection and time and frequency synchronization. The search process, called cell-search, is based on the special signals transmitted by the base stations, such as preambles. Once a suitable base station is found, the random access takes place, which typically includes two steps: (1) ranging and (2) resource request and allocation. During ranging, the mobile station sends a special signal to the base station so that the base station can identify the mobile station with certain associated parameters. During resource request and allocation, the uplink (UL) and downlink (DL) resources for communication are requested and allocated.
In a multi-carrier communication system, such as an Orthogonal Frequency Division Multiplex (OFDM) system, the signal in the time domain is generally organized into frames and each frame may consist of multiple OFDM symbols that carry the information. In order to acquire the multi-carrier signals correctly, the receiver is normally required to find the beginning of a frame. In an Orthogonal Frequency Division Multiple Access (OFDMA) network, both cell-search and ranging processes are critical parts of the protocol. In the cell-search process, a mobile station has to detect the special signal (e.g., the preamble) broadcast by a particular base station, whereas in the ranging process, a base station has to detect the ranging signal sent by a mobile station.
In the case of multiple antennas/receivers, the appropriate use of the multiple received signals can significantly benefit receiver functions such as frame synchronization, cell-search, ranging processes, and frequency offset estimation.